The present invention relates to an electrical contactor, particularly but not necessarily exclusively for moderate AC switching contactors employed in modern electricity meters, so-called ‘smart meters’, for performing a load-disconnect function at normal domestic supply mains voltages, typically being 100 V AC to 250 V AC. The invention may also relate to an electrical contactor of a moderate, preferably alternating, current switch which may be subjected to a short-circuit fault condition requiring the contacts to not weld. In this welded-contact fault condition, un-metered electricity is supplied. This can lead to a life-threatening electrical shock hazard, if the load connection that is thought to be disconnected is still live at 230 V AC. Furthermore, the present invention relates to a switch member for such an electrical contactor and/or to a method of controlling electrical contact closing and opening delay, thereby reducing contact erosion, arcing and/or tack welding.
Additionally, it is a requirement that the opening and closing timing of the electrical contacts in such a moderate-current switch should be more precisely controlled to reduce or prevent arcing damage thereby increasing their operational life.
It is known that many electrical contactors are capable of switching nominal current at, for example, 100 Amps, for a large number of switching load cycles. The switch contacts utilize a suitable silver-alloy which prevents tack-welding. The switch arm carrying the movable contact must be configured to be easily actuated for the disconnect function, with minimal self-heating at the nominal currents concerned.
Most meter specifications stipulate satisfactory nominal-current switching through the operational life of the device without the contacts welding. However, it is also required that, at moderate short-circuit fault conditions, the contacts must not weld and must open on the next actuator-driven pulse drive. At much higher related dead-short fault conditions, it is stipulated that the switch contacts may weld safely. In other words, the movable contact set must remain intact, and must not explode or emit any dangerous molten material during the dead-short duration, until protective fuses rupture or circuit breakers drop-out and disconnect the Live mains supply to the load. This short-circuit duration is usually for only one half-cycle of the mains supply, but in certain territories it is required that this short-circuit duration can be as long as four full cycles.
In Europe, and most other countries, the dominant meter-disconnect supply is single-phase 230 V AC at 100 Amps, and more recently 120 Amps, in compliance with the IEC 62055-31 specification. Technical safety aspects are also covered by other related specifications such as UL 508, ANSI C37.90.1, IEC 68-2-6, IEC 68-2-27, IEC 801.3.
There are many moderate-current meter-disconnect contactors known that purport to satisfy the IEC specification requirements, including withstanding short-circuit faults and nominal current through the operational life of the device. The limiting parameters may also relate to a particular country, wherein the AC supply may be single-phase with a nominal current in a range from 40 to 60 Amps at the low end, and up to 100 Amps or more recently to a maximum of 120 Amps. For these metering applications, the basic disconnect requirement is for a compact and robust electrical contactor which can be easily incorporated into a relevant meter housing.
In the context of the IEC 62055-31 specification, the situation is more complex. Meters are configured and designated for one of several Utilization Categories (UC) representing a level of robustness regarding the short-circuit fault-level withstand, as determined by certain tests carried out for acceptable qualification or approval. These fault-levels are independent of the nominal current rating of the meter.
Acting as an actuation means, there will typically be an armature or plunger which is driven to control the opening and closing of the contacts. However, a typical actuator can only provide an actuation in a single direction, which can cause problems in multi-pole contactors.
Some contactors utilize parallel or substantially parallel movable arms which are simultaneously actuated by a wedge-shaped member which is forced between them, separating the arms and breaking two contacts simultaneously. However, there can be an advantage to providing movable arms arranged in an anti-parallel configuration, to maximize repulsion forces between the arms to enhance the contact pressure when engaged. Such an arrangement however, cannot be achieved with an actuation in a single direction.
The present invention seeks to provide solutions to the afore-mentioned problems by providing a contactor having an actuator with a rocking armature.